Elucidating CO2 Hydrogenation over In2O3 Nanoparticles using Operando UV/Vis and Impedance Spectroscopies

Abstract In2O3 has emerged as a promising catalyst for CO2 activation, but a fundamental understanding of its mode of operation in CO2 hydrogenation is still missing, as the application of operando vibrational spectroscopy is challenging due to absorption effects. In this mechanistic study, we systematically address the redox processes related to the reverse water‐gas shift reaction (rWGSR) over In2O3 nanoparticles, both at the surface and in the bulk. Based on temperature‐dependent operando UV/Vis spectra and a novel operando impedance approach for thermal powder catalysts, we propose oxidation by CO2 as the rate‐determining step for the rWGSR. The results are consistent with redox processes, whereby hydrogen‐containing surface species are shown to exhibit a promoting effect. Our findings demonstrate that oxygen/hydrogen dynamics, in addition to surface processes, are important for the activity, which is expected to be of relevance not only for In2O3 but also for other reducible oxide catalysts.

S3 gases H2O and CO were integrated. The position of the maxima of the used bands are given in Table S1. The catalytic activity (CO2 conversion) is defined as the ratio of the amount of CO measured by the FTIR spectrometer at the outlet of the cell to the amount of dosed CO2. Based on our IR analysis, our selectivity for CO is 100%. To check the stability of the catalyst, we exposed the catalyst to reaction conditions (250 °C, 4 vol% H2, 2 vol% CO2, Ar) for over 5 hours but did not see any indication for instability or change in activity. Table S1: IR band maxima of gas phase H2O and CO used for the determination of the catalytic activity in the framework of the operando measurements.

CO 2174
Quasi in situ Raman Spectroscopy. Raman spectra were recorded on an HL5R transmission spectrometer (Kaiser Optical Systems Inc., USA), which employs a frequency-doubled Nd:YAG laser (Cobolt Inc., Hübner Photonics GmbH, Germany) for excitation at 532 nm, as described previously. [8,9] For the measurements, a ½ inch immersion probe with sapphire optics (Kaiser Optical Systems Inc., US) was applied to the reaction chamber. The stability of the band positions is better than 0.3 cm -1 . The laser power at the position of the sample was set to 6 mW, with a spot size approximately 30 μm in diameter. After gas treatment, the sample was cooled to 50 °C in argon (total flow: 100 mL min -1 ) for the Raman measurements, which were set to an exposure time of 900 s and two accumulations. For each measurement a cosmic ray filter was used, with a second spectrum recorded at each accumulation to eliminate spikes caused by cosmic rays, resulting in a total measurement time of about 1 h. The corresponding dark spectrum was recorded at the beginning of the measurement series. In this context, it should be mentioned that a separate dark spectrum before each individual measurement does not lead to any significant differences.

X-Ray Photoelectron Spectroscopy (XPS)
. XP spectra were recorded on a modified LHS/SPECS EA200 magnetic circular dichroism (MCD) system described previously. [1,2,7] The XPS system was equipped with a Mg Kα source (1253.6 eV, 168 W) and the calibration of the binding energy scale was performed with Au 4f7/2 = 84.0 eV and Cu 2p3/2 = 932.67 eV signals from foil samples. Prior to the measurements, the sample was treated with different gas atmospheres at 250°C, and the subsequent transfer of the sample into the analysis chamber was S4 performed without any air exposure (quasi in situ). Sample charging was compensated by setting the peak of the C 1s signal to 284.8 eV. Survey spectra were recorded at a resolution of 0.4 eV and narrow scans at a resolution of 0.025 eV. The O:In ratios in Figure S13 were obtained by integrating the In 3d5/2 and the O 1s signals after a Shirley background subtraction from the detailed spectra. The resulting areas were corrected with the corresponding relative sensitivity factors, i.e., 3.9 for the In 3d5/2 signal and 0.66 for the O 1s one. [10] Ultraviolet Photoelectron Spectroscopy (UPS). UP spectra were recorded on our modified LHS/SPECS EA200 MCD system (see XPS). As UV source we used a UVS 10/35 (SPECS) in combination with helium. The gas inlet was controlled by a differential pumping system Operando Impedance Spectroscopy. Potentiostatic electrochemical impedance spectra (p-EIS) were acquired in a two-electrode system using a BioLogic VSP potentiostat/galvanostat operated in the 1 MHz to 10 Hz range with 50 mV amplitude and 10 measurements points per decade acquired in triplicate in a potential range of 0.05-1 V versus a reference of +0.338 V (In 3+ to In 0 ). [11] The positive reduction potential was referenced against the standard hydrogen electrode (SHE) and the potential interval was set to dE = 0.095 V. Impedance spectra were recorded in a commercial CCR1000 cell (Linkam Scientific Instruments, UK), equipped with a PTFE plate with two holes for the copper electrodes (see Scheme S1). In this context, we also performed experiments with gold electrodes (Alfa Aesar, UK, 99.999 %), where no influence of electrode material on the electrochemical output was observed with the exception of a parasitic potential iR drop, arising from the peculiarities of cell assembly. Its compensation was performed manually in the EC-Lab v. 11.33 (BioLogic, France) acquisition software prior to S5 the actual measurement. Before each measurement, the sample was kept at 250 °C for about 40 min in 25 vol% O2, 4 vol% H2, 2 vol% CO2, or a mixture of H2/CO2 (4 vol%/2 vol%) balanced in argon at a total flow rate of 50 mL/min for equilibration. This procedure ensures that the measurements take place in a stationary state. This is verified by considering the temporal evolution, which for the respective potential did not show any significant changes during the measurement. Raw spectra were validated by applying the Kramers-Kronig relations, which deviate from ideal behavior by ca. 10%, with 11% being the benchmark for discarding the measurement, meaning that the real and imaginary parts of the experimental spectra overlap with the e.g. imaginary spectral points calculated from e.g. real part by applying Hilbert transforms. The impedance models were built in order to fit the acquired p-EIS spectra by the numerical Fortran-assisted Z-fit procedure (EC-Lab v. 11.33). For the fit analysis a numerical randomization followed by the Levenberg-Marquardt algorithm was utilized, while the number of iterations was kept at 500k to ensure full conversion of the equivalent-circuit model (error threshold: χ/|Z| < 0.5).
At the same time as the impedance measurements, the gas phase was measured using a FTIR spectrometer (Tensor 20, Bruker). The settings correspond to those of the operando UVvis measurements. The CO2 conversion during the reaction phases was estimated to be 4 %.
Scheme S1. Scheme of the experimental setup used for the impedance measurements. RE: reference electrode, WE: working electrode, CE: counter electrode. For more details see text. Table   Figure   after the gas exposure, except for the last O2 phase, which was recorded after 30 min. The hyphenated numbers indicate how often the sample was exposed to the respective gas phase.

Supplementary Figures and
S7 Figure S3. XRD results of c-In2O3 sheets recorded after the indicated gas exposures at 250 °C (total flow rate: 100 mL min -1 ). The diffraction patterns were recorded sequentially in the following order: O2, H2, CO2, H2, H2/CO2. The diffraction pattern after the CO treatment was recorded after a prior O2 treatment. The hyphenated numbers in the legend indicate how often the sample was exposed to the respective gas phase. Figure S4. In situ UV-vis spectra for In2O3 sheets recorded during the indicated gas exposures at 250 °C and at a total flow rate of 100 mL min -1 . Spectra were recorded about for 1 h after the gas exposure. The hyphenated numbers in the legend indicate how often the sample was exposed to the respective gas phase.     Figure S6. The experimental data shows a reversed behavior, where highfrequency points are located in the right part of the spectrum. R: resistance, C: capacitance, L: inductance. Q: non-ideal capacitance.

Discussion of the quasi in situ XPS results
Figure S13 depicts the In 3d photoemissions, recorded after exposure to O2, H2, H2/CO2, and CO2. After reaction conditions, a slight blue-shift in the binding energy (0.2 eV) and after the second H2 pretreatment a slight red-shift are observed. The blue-shift may be indicative of the formation of surface hydroxides during reaction conditions and would be consistent with the findings from Raman spectroscopy, whereas the red-shift points to the formation of metallic indium species, indicating that metallic indium is also formed in small amounts at the surface besides its presence in the bulk, as shown by p-EIS and XRD analysis (see Fig. S2). These changes in the In 3d signals are also reflected in the corresponding linewidths (see Figure S15), revealing a significant increase in the FWHM after reaction conditions and the second H2 exposure.
From the O:In ratios, shown in Figure S15, it can be seen that the surface is reduced under H2 and oxidized under CO2 (and O2) atmosphere, in analogy to our other findings (see Figure   2). We attribute the observed behavior to variations in the number of oxygen vacancies and note that within the experimental uncertainty we do not detect differences between exposure to the CO2 and O2 phases. Finally, it should be mentioned that no significant changes can be seen in the O1s signals (see Figure S14A).